Nitrogen oxide storage catalytic converters are used to remove the nitrogen oxides contained in the lean exhaust gas of so-called lean-burn engines. Here, the purification effect is based on the fact that, in a lean-burn operating phase (storage phase, lean-burn mode) of the engine, the nitrogen oxides are stored by the storage material of the storage catalytic converter in the form of nitrates. In a subsequent rich-burn operating phase (regeneration phase, rich-burn mode) of the engine, the previously-formed nitrates are broken down and the released nitrogen oxides are converted with the rich constituents, which have a reducing action, of the exhaust gas during the rich-burn mode at the storage catalytic converter to form nitrogen, carbon dioxide and water. Rich constituents of the exhaust gas include hydrocarbons, carbon monoxide and hydrogen.
The mode of operation of nitrogen oxide storage catalytic converters is described in detail in the SAE document SAE 950809. The composition of nitrogen oxide storage catalytic converters is sufficiently well-known to a person skilled in the art. The nitrogen oxide storage materials are generally alkaline compounds of alkali or earth alkali metals such as, for example, oxides, hydroxides or carbonates of barium and strontium which are applied in finely distributed form to suitable support materials. In addition, a nitrogen oxide storage catalytic converter also has catalytically active noble metals of the platinum group and oxygen storage materials. Said composition gives a nitrogen oxide catalytic converter the function of a three-way catalytic converter under stoichiometric operating conditions.
The storage phase (lean-burn mode) usually lasts 100 to 200 seconds and is dependent on the storage capacity of the catalytic converter and the concentration of the nitrogen oxides in the exhaust gas. In the case of aged catalytic converters with reduced storage capacity, the duration of the storage phase can however also fall to 50 seconds and less. The regeneration phase (rich-burn mode) is, in contrast, always significantly shorter and lasts only a few seconds. The exhaust gas emerging from the nitrogen oxide storage catalytic converter during regeneration is substantially free of pollutants, and is of stoichiometric composition. Its air ratio λ is approximately equal to 1 during said time.
At the end of the regeneration phase, the released nitrogen oxides and the oxygen which is attached to the oxygen storage components of the catalytic converter are no longer sufficient to oxidize all the rich exhaust gas constituents. There is therefore a breakthrough of said constituents through the catalytic converter, and the air ratio falls to a value below 1. Said breakthrough indicates the end of the regeneration and can be registered by means of a so-called lambda probe downstream of the storage catalytic converter.
The regeneration of a storage catalytic converter is therefore inevitably associated with short emissions peaks of hydrocarbons and carbon monoxide, which are intensified yet further by the following effect: the exhaust line between the engine and storage catalytic converter usually has a finite length. During the switch from the rich-burn mode into the lean-burn mode, the entire exhaust line is still filled with rich exhaust gas, which is discharged via the catalytic converter into the environment after the switch by the now-lean exhaust gas of the engine. The catalytic converter can no longer purify said residual proportion of rich exhaust gas, since the previously stored, oxidizing constituents were already consumed during the rich-burn mode. There is therefore a resulting intensified emissions peak of hydrocarbons and carbon monoxide at the end of a regeneration.